On the Chief Line in the Spectrum of the Nebulcp. 185 



also used for adjusting the object-glass at the proper distance from 

 the slit. The ordinary -cross- wire eyepiece of the spectroscope being 

 replaced by the bright line micrometer, the prisms were adjusted at 

 minimum deviation for X, 500 by observing the spectrum of mag- 

 nesium ribbon burning in a spirit lamp in front of the centre of the 

 mirror. These preliminaries being completed, the work with the 

 nebula was commenced. 



The nebula -was first brought upon the slit by means of the 

 reflecting eyepiece, and the observation was attempted with four 

 prisms, but unsuccessfully, as the night was not good and the nebula 

 was low, so two were removed. When this was done, the three prin- 

 cipal lines were seen remarkably well, and a very narrow slit could be 

 used. The chief line was made coincident with the illuminated 

 pointer ; magnesium ribbon was then burned at the centre of the 

 mirror, and with this dispersion the coincidence between the nebula 

 line and the least refrangible maximum of the magnesium fluting 

 appeared perfect. These observations were made independently by 

 Messrs. Fowler and Baxandall, and Lieutenant Bacon, R.N\, tempo- 

 rarily attached to the Science Schools, but in no case was the 

 nebula line seen more refrangible than the magnesium fluting. 

 Another prism was then added, and set to minimum for X 500. 



Absolutely the same result was obtained. The burning magnesium 

 used for comparison was removed from the 'front of the mirror and 

 placed directly in front of the slit, but still the same result was 

 obtained. The brightness of the nebula lines with three prisms made 

 it evident that another prism might be added. 



The magnifying power of the telescope employed was sixteen, and 

 the dispersion C to H with three prisms was 6 32'. 



November 28th. The observations of the Orion nebula were 

 repeated with similar arrangements to those employed on the previous 

 evening, the fourth prism being now added. The spectrum was very 

 well seen when the nebula was on the slit, but it was very difficult to 

 keep it on, as, in consequence of the looseness of a screw, as it was 

 subsequently found, the siderostat clock worked badly. 



One comparison was, however, made by Mr. Fowler, using the 

 same micrometer eyepiece as before and a very narrow slit. The 

 nebula line and the less refrangible maximum of the magnesium 

 fluting were found to be perfectly coincident. 



Arrangements had been made during the day for burning mag- 

 nesium, so as to get parallel rays from it. The method is shown in 

 fig. 2, and consists of a collimator placed in front of the object- 

 glass. When burning the magnesium, a card, with a hole in the 

 centre of the same diameter as the collimating lens, was placed in 

 front of the 10-inch object-glass to keep out stray light. At the 

 spirit lamp end of the tube was a piece of tin foil with a pin-hole at 



o 2 



